One type of positive displacement flowmeter uses several evenly spaced, radially directed pistons to measure fluid flow through the meter. The connecting rods of the pistons are coupled to one another in various manners, such as with the pivoting link disclosed in U.S. Pat. No. 168,354 to Swasey. The cylinders are fluidly connected to one another and to the entry and exit ports through various passageways. The operation of such radial piston flowmeters is typically monitored through the use of a rotating indicator shaft connected to the link mechanism coupling the crankshafts by an eccentric pin. Measuring the number of rotations of the output shaft allows the volume of fluid passing through the meter to be determined.
Significant problems arise when such a meter is to be used at low flow rates and/or with low viscosity fluids with poor lubricating qualities.
One problem is that the pressure drop across a meter must be kept low otherwise fluid will slip by the metering pistons and not contribute to their total displacement. For example, a meter with a 3/4" bore and a 3/4" stroke running at 10 cc/min should have a pressure drop of a few hundredths of a psi at most (to produce an inaccuracy of a few percent or less).
The second problem is that a liquid film is easily squeezed out of the space between sliding components. With a film present wear is almost nonexistent and sliding friction is very low. When such a film is squeezed out and a component such as a piston touches the cylinder wall, wear results and friction is many times higher resulting in higher pressure drop and typically rough meter operation.
The four piston design of the Swasey meter uses a link between connecting rods. This particular design has the advantage of simplicity but has a significant drawback. Each time a pair of pistons are in a position of reversal (top dead center) forces can be applied to the remaining two sliding pistons which are normal to their motion. If the meter is running very slowly (2 or 3 cycles per minute) forces normal to the piston direction of travel tend to squeeze the fluid film from between the piston and cylinder wall. A meter constructed of stainless steel using chrome plated stainless pistons metering water may lock up once this occurs.
There are three factors which are critical to this design being useful in low flow rate applications for low viscosity non-lubricating fluids.
First, the pistons and pivoting link must move very freely. This means they must not be subjected to twisting or off center or binding forces, and as little energy as possible must be extracted from their motion.
Second, the clearances of piston and cylinders must be close. Third, the selection or materials for pistons and cylinders is critical in low flow rate applications.
All prior art meters require the pivoting link to provide rotational energy to the readout device. This could be a counter or an electrical pulse generator. This approach requires several bearing surfaces (3 internal to the meter and 2 or more external to the meter) in addition to the two required for piston synchronization. The extra mechanism results in significant extra frictional forces in the meter and quite a number of mechanical mechanisms.